Known in the art is an exhaust purification system of an internal combustion engine provided with an exhaust purification catalyst arranged in an exhaust passage of the internal combustion engine and able to store oxygen and an air-fuel ratio sensor or oxygen sensor arranged at an upstream side and downstream side in this exhaust purification system in a direction of flow of exhaust (for example, PTLs 1 to 4). In such an exhaust purification system of an internal combustion engine, an air-fuel ratio detected by the upstream side air-fuel ratio sensor (below, referred to as an “output air-fuel ratio”) is controlled to a target air-fuel ratio by feedback control of the amount of feed of fuel from a fuel injector.
Among these, in the exhaust purification system described in PTL 1, the output air-fuel ratio of the downstream side air-fuel ratio sensor is used as the basis for control of the target air-fuel ratio. Specifically, if the output air-fuel ratio of the downstream side air-fuel ratio sensor becomes a rich judged air-fuel ratio richer than the stoichiometric air-fuel ratio or becomes less, the target air-fuel ratio is switched to a lean air-fuel ratio leaner than the stoichiometric air-fuel ratio. In addition, if the output air-fuel ratio of the downstream side air-fuel ratio sensor becomes a lean judged air-fuel ratio leaner than the stoichiometric air-fuel ratio or becomes more, the target air-fuel ratio is switched to a rich air-fuel ratio richer than the stoichiometric air-fuel ratio. According to PTL 1, due to this, it is considered possible to suppress the outflow of unburned fuel or NOx etc. from the exhaust purification catalyst. In addition, by making the oxygen storage amount of the exhaust purification catalyst vary between a maximum storable oxygen amount (maximum amount of oxygen which the exhaust purification catalyst can store) and zero, the oxygen storage ability of the exhaust purification catalyst can be maintained high.